1. Field of the Invention
This invention relates generally to blow molding and in particular to so called “extrusion” blow molding where a parison from which an article is to be blow molded is produced by extrusion of material in a flowable condition through a forming die and the extruded parison is expanded within a mold cavity by introduction of pressurized fluid into the parison.
2. Description of Related Art
Blow molding is a cyclic process wherein a parison of material in a flowable condition is centered on the parting plane of a mold assembly defining a mold cavity, the mold assembly is closed around the parison, the parison is expanded so the exterior of the parison abuts molding surfaces of the mold cavity, the blow molded article is conditioned to be sufficiently rigid to retain its size and shape while unsupported, the mold assembly is opened and the blow molded article is removed. In general, the flowable condition of the parison is achieved when the material of the parison is at an elevated temperature, and the rigid condition of the blow molded article is achieved when the temperature of the material of the article has been reduced. A blow molding machine comprises a press for opening and closing the mold assembly and blowing means for introducing a pressurized fluid (typically air) to expand the parison within the mold cavity. In so called extrusion blow molding, a tubular parison is formed by passing material in a flowable condition through a forming die and the mold assembly is closed on the tubular parison so as to close the parison except for an opening for introduction of pressurized fluid.
To introduce pressurized fluid into an extruded parison, a so called blow pin is introduced through the opening in the parison, the blow pin comprising a passage there through in fluid communication with a source of pressurized fluid and with the interior of the parison. The blow pin seals the opening of the parison so as to prevent pressurized fluid bypassing the blow pin. The blow pin is attached to a blow pin shaft passing through an opening through the forming die interiorly of the channel through which material is extruded and rigidly connected to an actuator rod to effect translation of the blow pin shaft. It is known to provide adjusting devices affecting the lateral position of the blow pin shaft relative to the centerline of the mold cavity. However, in known constructions as described, lack of parallelism between the centerline of the forming die and the blow pin shaft can result in misalignment of the blow pin at the opening of the parison. Such misalignment can result in damage to the blow pin, perforation of the parison where the blow pin forms a seal with the parison or an inadequate seal permitting pressurized fluid to by-pass the blow pin causing incomplete expansion of the parison. Further, misalignment can cause binding of the blow pin shaft with the opening through which it passes as the blow pin shaft is translated between a retracted position and an advanced position. Lack of parallelism may result from, for example, a rod of an actuator for moving the blow pin not being perpendicular to the actuator rod bearings; a mounting face of the actuator not being flat and/or perpendicular to the actuator rod and actuator rod bearings; support of the actuator mounting not being perpendicular to the centerline of the forming die; lack of concentricity of the blow pin shaft with the actuator rod as the blow pin shaft is connected to the actuator rod; lack of concentricity of the blow pin relative to the blow pin shaft as the blow pin is supported on the blow pin shaft; and, lack of straightness of the blow pin shaft. Hence, there is a need for an improved construction to achieve parallelism of the blow pin shaft with the centerline of the opening through the forming die and to achieve desired alignment of the blow pin with the opening of the parison.